1. Field of the Invention
This invention relates to an evaporative fuel-purging control system for an internal combustion engine having an evaporative emission control system.
2. Prior Art
Conventionally, evaporative emission control systems have been widely used in internal combustion engines, which operate to prevent evaporative fuel (fuel vapor) from being emitted from a fuel tank into the atmosphere, by temporarily storing evaporative fuel from the fuel tank in a canister, and purging same into the intake system of the engine. Purging of evaporative fuel into the intake system causes instantaneous enriching of an air-fuel mixture supplied to the engine. If the purged evaporative fuel amount is small, the air-fuel ratio of the mixture will then be promptly returned to a desired value, with almost no fluctuation.
However, if the purged evaporative fuel-purging amount is large, the air-fuel ratio of the mixture fluctuates. To cope with this, there has been proposed by the assignee of the present invention an evaporative fuel-purging control system which includes a plurality of flowmeters arranged across a purging conduit line connecting between a canister and an intake system of an internal combustion engine and detects a flow rate of a mixture of evaporative fuel and air flowing in the purging conduit line, a flow rate of evaporative fuel, etc., based upon output values of the flowmeters (Japanese Patent Application No. 3-80726).
This proposed system further includes a purge control valve arranged across the purging conduit line, which is driven to control the flow rate of the mixture such that the actual flow rate of evaporative fuel becomes equal to a desired value.
However, if any of the flowmeters becomes defective, the proposed system cannot perform accurate control of the flow rate of evaporative fuel supplied to the engine intake system. For example, an excessive amount of evaporative fuel can be supplied to the intake system, resulting in overriching of the air-fuel ratio. Conversely, an insufficient amount of evaporative fuel can be supplied to the intake system, resulting in overleaning of the air-fuel ratio. This leads to degraded exhaust emission characteristics, etc.